Recently, in accordance with the flattening of a front panel of a color cathode ray tube, a shadow mask has also been flattened. As the shadow mask is flattened, a flat surface of the shadow mask cannot be maintained only by supporting a shadow mask main body by a frame as in the prior art. Furthermore, when the shadow mask is merely supported by the frame, the shadow mask is easily subjected to the vibration from the outside, affecting a display screen of the color cathode ray tube. Therefore, the shadow mask is stretched under a constant tension.
On the other hand, in a doming phenomenon in which electron beams collide into the shadow mask to effect a thermal expansion and to deform the surface of the shadow mask, the amount of shift of the electron beams due to the doming is increased especially in the neighborhood of the both ends of a display screen. Therefore, when the shadow mask is stretched and fixed, in order to absorb the thermal expansion due to the collision of the electron beams, the maximum practical level of tension, which is almost the elastic limit, is applied to the shadow mask.
According to such a stretching and fixing, even when the temperature of the shadow mask is increased, the vibration of the shadow mask due to the external vibration and relative shift between electron beam through apertures of the shadow mask and fluorescent dots on a fluorescent screen can be inhibited. A shadow mask that is stretched and fixed is referred to as a tension type shadow mask. Examples of the tension type shadow mask include an aperture grill type shadow mask in which a large number of thin wire materials is stretched and fixed to a mask frame; a slot type shadow mask in which a flat plate is provided with a large number of approximately rectangular electron beam through apertures; and a dot type shadow mask in which a flat plate is provided with a plurality of round shaped electron beam through apertures.
Furthermore, the stretching and fixing method includes a one-dimensional tension method and a two-dimensional tension method. In the one-dimensional tension method, a tension is applied to only the longitudinal direction (vertical direction). On the other hand, in the two-dimensional tension method, a tension force is applied to both the longitudinal direction and width direction. The aperture grill type shadow mask uses the one-dimensional tension method; and the slot type or dot type shadow mask uses the one-dimensional tension method or the two-dimensional tension method.
In order to apply a predetermined tension force to the shadow mask, various kinds of methods have been suggested, in which a tension force is applied to a shadow mask and the shadow mask and a mask frame are welded and fixed with each other in a state in which a compression force is applied to the mask frame (See for example, JP-A No. Sho 63-298936, JP-A No. Hei 8-55577 and JP-A No. Hei 9-7505).
Furthermore, JP-A No. Hei 9-7508 discloses a method in which the compression force is applied to the substrate side of the mask frame.
However, the above-mentioned methods for manufacturing a color cathode ray tube according to the prior art have the following problems.
(1) In a case where the compression force is applied to many places on the mask frame, the distribution of the compression force is not uniform and the difference in the compression force is increased between the portion to which the compression force is applied and the other portion, and thus the frame is deformed. Therefore, when the compression force is released after welding, wave-like deformation of the frame is reflected on the tension force of the shadow mask, causing wrinkles on the shadow mask.
(2) In the method for applying the compression force to the substrate side of the mask frame, the compression force is released after the shadow mask and mask frame are welded and fixed to each other, a sufficient repulsion force is not applied to the upper end of the mask frame, and thus a predetermined tension force cannot be maintained.